1. Field of the Invention
This invention pertains to the field of television picture tubes and to a device for correcting residual mis-convergence errors in the cathode ray tube (CRT) or picture tube. More particularly, the invention concerns a novel device for adding to the deflection yoke in order to correct residual mis-convergence of the green, red and blue beams as they strike the pixels located on the inside of the face of the tube.
2. Description of the Prior Art
A cathode ray tube (CRT) or television tube generally comprises a narrow-necked section, located at the rear of the tube, in which electron guns mounted therein generate forwardly-directed electronic beams, an outwardly-opening center skirt section, where the beams are spread outward by magnetic forces, and a relatively flat, specially coated, viewing screen section at the front of the tube, lying generally orthogonal to the axis of the narrow-necked section and upon which the spreading beams strike to produce images (the television picture) for viewing from in front of the tube. The tube operates by directing the electronic beams of energy forward through the neck of the tube and flaring them onto the coating, known as phosphors or pixels, coated on the inside of the face or viewing screen located at the front of the tube. Color television sets and computer monitors use such a CRT and the pixels but require three different beamsxe2x80x94blue, green and red. When these beams converge on the pixels they produce a color that is viewable from the front of the CRT, and this is the color that is seen by the viewer sitting in front of the television set or computer.
The three beams generated in guns located centrally in the narrow-necked section at the rear of the tube are initially maintained converged in the neck of the tube. The beams are then deflected and converged at all other points of the screen by a device known as a deflection yoke (DY). Typically, a deflection yoke consists of pairs of electromagnetic coils energized by electric currents to create magnetic fields for deflecting the beams in the horizontal direction and vertical direction. The coils comprise wound loops of small diameter copper wire. The pairs of coils are nestled in a plastic liner (or separator) which also serves to electronically insulate the two pairs of coils from one another. The DY controls the individual paths of the three beams as they traverse the screen, beginning at the top left corner of the screen and traveling across to the top right corner then repeating this travel on the next, lower line of pixels below that previously traveled and continue back and forth until the entire screen has been sprayed with the beams. This traversing action is accomplished hundreds of times per second, faster than can be discerned by the human eye, and thus is presented to the viewer as a complete screen full of colored objects moving about as in a play, a dance or a motion picture show.
While the electromagnets located on the separator are sufficient, both in intensity and in operation, to control the paths of the three beams generally in the center of the tube and throughout much of the flaring action, shifts occur in the paths of the three beams as they approach the extreme edges of the screen. This deflection results in some misalignment and mis-convergence of the three beams at the edges of the picture tube, dulling of the color and focus and generally degrading performance of the picture tube from producing high quality reproduction of what is intended to be presented.
At present, the prior art deals with this problem by having a workman energize the picture tube during the latter stages of television manufacture and assembly but after the picture tube has been totally constructed. He or she visually observes the misalignment or mis-convergence of the beam paths outward from the center of the tube. He or she then temporarily removes the yoke from around the narrow-necked portion of the tube, reaches a hand into the inner surface of the wire-wound coils, and applies small, short, flat strips of plastic ferrite, each having a high density of magnetic particles embedded therein, to the inner surface of the coils and covers the strips with adhesive tape. The tube, and yoke are then reassembled. These small strips later operate to distend the magnetic fields generated by the large electromagnetic coils in the yoke and this distention is intended to correct the mis-convergence visually perceived by the assembler.
The number and location of these small strips of xe2x80x9cmagneticxe2x80x9d tape are determined by the expertise of the person doing the testing and adding the tape to the magnets on the assembly line and carry the overriding problems of worker fatigue, off-the-job sickness, human temperament, and the like. In addition, removing and then replacing the yoke, after application of the small strips of tape, is a time-consuming practice that adds unnecessary cost to the television set assembly process resulting in lost profits.
An alternative prior art practice involves connecting small xe2x80x9cauxiliaryxe2x80x9d coils in series or parallel with the main yoke coils in different circuit configurations in such a way that these auxiliary coils provide an additional magnetic field necessary to correct mis-convergence errors. Typically, these auxiliary coils are adjusted by a potentiometer or position of a ferrite core within the coil. Obviously, this is an expensive and time-consuming solution.
This invention is a device for correcting residual mis-convergence errors in a color cathode ray tube that may be assembled with the tube and later used to correct the mis-convergence without disassembling the tube or adding tape or extra wire-wound electromagnets to the tube assembly. The invention comprises a deflection yoke liner (separator) of an injection-molded plastic wherein the plastic includes a high density of magnetizable particles therein, where the separator is mounted about the CRT. A plurality of small, powerful magnets are formed in the magnetizable particles in the separator by application of short-duration, high-voltage pulses applied thereto from outside sources. The magnets are created and positioned on the separator from knowledge about the correlation between the magnets"" location and maximum resulting effects on mis-convergence errors in the tube viewing screen and applying appropriate correction to the strength of the magnets.
The process of using the separator of this invention, to correct mis-convergence of electron beams in a color picture tube, includes the steps of first affixing the separator, containing at least one pair of deflection coils, about the picture tube, energizing the guns located inside the tube to generate the forwardly-directed electron beams, adjusting the electromagnetic deflection coils to obtain a tightly focused convergence of the beams as practical, measuring the residual mis-convergence errors on the screen, temporarily applying a plurality of small, electric magnetizing coils to various locations around the separator, wherein the locations are determined by application of an algorithm to counteract mis-convergence of the electron beams at locations not corrected by the larger electromagnetic coils, and applying iteratively single applications of short-duration, high voltage pulses to create small magnetic areas in the separator that will correct all the residual mis-convergence errors throughout the screen. The result is a CRT that does not require disassembling during production or the addition of hand-placed strips of magnetic tape or other handling that results in prolonged production time and loss of profits.
Accordingly, the main object of this invention is to streamline the final stages of correcting mis-convergence of the red, green and blue electron beams generated in the guns in the aft end of the picture tube so that the present practice of applying small bits of magnetic distorting tape to the large electromagnetic coils is eliminated. Other objects of the invention include a method that removes the human element from correcting mis-convergence of the electron beams in a CRT and replaces it with an automatic system that provides consistent results in a continuous manner not adversely affected by the fragilities of human intervention; a means of reducing assembly time in the color CRT industry by eliminating the step of dismantling the yoke on the neck section of a CRT; and a means of eliminating the need for specially trained, heavily experienced personnel in the steps of correcting mis-convergence in CRT assembly.
These and other objects of the invention will become more clear when one reads the following specification, taken together with the drawings that are attached hereto. The scope of protection sought by the inventors may be gleaned from a fair reading of the Claims that conclude this specification.